1. Field of the Invention
The present invention relates to a spread-signal multiplexing circuit for multiplexing a plurality of spread signals that were generated in parallel according to the SSMA (spread spectrum multiple access) scheme.
2. Description of the Related Art
The CDMA scheme is now positively applied to mobile communication systems increasingly because it is inherently superior in secrecy and resistance to interference, it enables flexible adaptation to multimedia and a variety of channel allocation forms, and a transmitting power control technique that can solve the near-far problem has been established recently.
In such mobile communication systems, the number of terminals where a call may occur and that exist in the wireless zone of a radio base station always varies depending on the time zone and other factors. In general, the above number increases from immediately after a start of operation of each radio base station or a corresponding mobile communication system.
In view of the above, the transmission system of each radio base station is equipped with spread-signal multiplexing circuits to enable both of flexible addition to accommodate increase in the number of terminals that may exist in the wireless zone of the radio base station and sharing of an antenna system to be used for transmission.
FIG. 8 shows the configuration of an exemplary conventional spread-signal multiplexing circuit.
As shown in FIG. 8, a plural number n of spread signals that were generated in parallel are supplied to the inputs of delay parts 91-1 to 91-n and filters 92-1 to 92-n, respectively. The outputs of the delay parts 91-1 to 91-n are connected to the inputs of filters 94-1 to 94-n via amplitude controlling parts 93-1 to 93-n, respectively. The outputs of the filters 94-1 to 94-n are connected to the inputs of transmitting parts 96-1 to 96-n via frequency converters 95-1 to 95-n, respectively. The outputs of the transmitting parts 96-1 to 96-n are connected to the corresponding inputs of a combiner 97. The output of the combiner 97 is connected to an antenna system (not shown) via a power amplifier 98. A plural number n of local frequency signals that correspond to bands where occupied bands of the n spread signals should be allocated on the frequency axis, respectively, and have different frequencies f1 to fn are supplied to the local frequency inputs of the frequency converters 95-1 to 95-n, respectively.
The outputs of the filters 92-1 to 92-n are connected to the inputs of frequency converters 99-1 to 99-n, respectively. The outputs of the frequency converters 99-1 to 99-n are connected to the corresponding inputs of a combiner 100. The output of the combiner 100 is connected to the coefficient inputs of the amplitude controlling parts 93-1 to 93-n via a coefficient calculating part 101. The above-mentioned n local frequency signals are supplied to the local frequency inputs of the frequency converters 99-1 to 99-n, respectively.
In the spread-signal multiplexing circuit having the above configuration, the filters 94-1 to 94-n have passages bands that are equal to the occupied bands of the n spread signals and the filters 92-1 to 92-n have passages bands that are somewhat wider than the passages bands of the filters 94-1 to 94-n.
The frequency converters 99-1 to 99-n generate a plural number n of intermediate frequency signals (hereinafter referred to as “subintermediate frequency signals”) in parallel by shifting, on the frequency axis, the occupied bands of the n spread signals to different bands corresponding to the frequencies f1 to fn, respectively. The combiner 100 combines those subintermediate frequency signals into a subtransmission wave signal.
On the other hand, holding propagation delay times that are equal to those of the parts from the inputs of the filters 92-1 to 92-n to the output of the coefficient calculating part 101 via the frequency converters 99-1 to 99-n and the combiner 100, respectively, the delay parts 91-1 to 91-n delay the n spread signals by those propagation delay times, respectively, in parallel. In the following description, for the sake of simplicity, spread signals that are output from the delay parts 91-1 to 91-n in parallel are called delayed spread signals, respectively.
The amplitude controlling parts 93-1 to 93-n limit (weight) the amplitudes of the delayed spread signals as appropriate chip by chip in accordance with a coefficient that is calculated by the coefficient calculating part 101. In the following description, for the sake of simplicity, spread signals that are output from the amplitude controlling parts 93-1 to 93-n in parallel will be called “shaped spread signals,” respectively.
The filters 94-1 to 94-n, the frequency converters 95-1 to 95-n, the transmitting parts 96-1 to 96-n, and the combiner 97 generate a transmission wave signal by frequency-multiplexing the n shaped spread signals. The generated transmission wave signal is supplied to the antenna system via the power amplifier 98. Pieces of processing that are performed by the individual parts during the above frequency multiplexing are basically the same as those that are performed by the filters 92-1 to 92-n, the frequency converters 99-1 to 99-n, and the combiner 100, and hence will not be described.
The coefficient calculating part 101 detects the peak value of the instantaneous value of the subtransmission wave signal chip by chip. For example, the coefficient calculating part 101 calculates, one by one, a coefficient z that is equal to the reciprocal of a ratio of the peak value to a known upper limit value that the peak value is allowed to have or the reciprocal of the square of the ratio.
The amplitude controlling parts 93-1 to 93-n generate n shaped spread signals by multiplying the instantaneous values of the n delayed spread signals by the common coefficient z, respectively.
With the above processing, even if the instantaneous values of the n spread signals become too large in any chip-based combination, the instantaneous value of the transmission wave signal that is supplied to the power amplifier 98 does not have such a large value that a sufficient level of linearity cannot be secured in the power amplifier 98. Therefore, a transmission wave signal that is actually supplied to the antenna system is prevented from containing undesirable spurious components, effective use of radio frequencies is enabled, and the transmission quality and the service quality are kept high.
Incidentally, in the above conventional example, the peak value of the instantaneous value of the transmission wave signal (or subtransmission wave signal) is proportional to the square of the multiplicity n and its average power is proportional to the multiplicity n.
That is, the peak factor (defined as a ratio of a maximum power to an average power or a ratio of a maximum value to an average value of instantaneous values) of the transmission wave signal (or subtransmission wave signal) increases as the multiplicity n increases. Therefore, spurious components occur undesirably in the power amplifier 98 and it is highly probable that such spurious components are emitted from the antenna system.
Such spurious components can be avoided by giving sufficient output back off to the power amplifier 98. However, in general, such output back off is attained by setting the saturation output level of the power amplifier 98 large and thereby making the level of a transmission wave signal that is output from the power amplifier 98 small relative to the saturation output level. Therefore, increasing the output back off is a factor of lowering the total power efficiency to a large extent, and the output back off is not employed in practice.
In general, the multiplicity n is necessarily set at a large value in such a radio base station that a lot of terminals may concentrate in its wireless zone and many calls may occur in those terminals in parallel.
Further, since the amplitude controlling parts 93-1 to 93-n adjust the amplitudes of n delayed spread signals discretely chip by chip, respectively, the chip-by-chip amplitude of a subtransmission wave signal may become too large in a one-shot-like manner (or sporadically). In such a case, it is highly probable that in the filtering processing that is performed by each of the filters 94-1 to 94-n an undue variation occurs in the amplitudes of chips adjacent to such a chip.
Where all or part of the components shown in FIG. 8 are implemented as digital signal processing, in such digital signal processing larger round-off error occurs when the multiplicity n or the peak factor is larger even if a subject of operation is scaled in advance as numerical information that gives a sufficient level of linearity in the dynamic range of the amplitude of a subtransmission wave signal or a transmission wave signal.
Therefore, unless a sufficient amount of processing to allow increase in the word length of the subject of operation is secured, it is highly probable to cause attendant technical problems such as an undue elevation of the noise floor of a transmission wave signal.